1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital copier, and in particular relates to an image forming apparatus that is provided with a post-processing unit installed in an internal space for discharging sheets of paper.
2. Related Art
With a conventional image forming apparatus utilizing electrophotography, heat and pressure are applied to a sheet of paper so as to fix toner transferred on it during a fixing process, so that the temperature of the sheet of paper tends to go beyond 100 degrees Celsius. It is normal that a sheet of paper always contains the moisture of a few percent and the moisture tends to gradually increase during storage subsequent to its production. In particular, after a wrapping paper covering a stack of paper is removed, the moisture percentage of the sheet of paper will increase remarkably.
The moisture contained in the sheet of paper rapidly evaporates when the heat and pressure are applied to the sheet of paper during the fixing process. In this process, the local difference in the moisture percentage occurs between the front surface and the back surface of the sheet of paper, and so on. In addition, there are a spot filled with and the other spot not filled with the moisture evaporated from the sheet of paper in the vicinity of a fixing unit, in particular, an area in the vicinity of the front surface and the back surface through which the sheet of paper travels. The sheet of paper with a high temperature, which has just been discharged from the fixing unit, travels through this area having the nonuniformity of moisture distribution and is carried to an outlet.
In this manner, it may be that the sheet of paper having received heat and pressure during the fixing process suffers from curls (curl habit). FIG. 17(a) is a perspective view illustrating sheets of paper 101 being stacked on a discharging tray, and FIG. 17(b) is a view on arrow illustrating one of the sheets of paper 101 shown in FIG. 17 (a), as seen in a direction of arrow Y. As shown in FIGS. 17(a) and 17(b), the sheets of paper 101, which have been discharged along a discharging direction of paper (a direction of arrow X in FIG. 17(a)), have remarkable curls at left and right end portions 101a of the sheets of paper 101. As an amount of curling varies according to the pressure applied to the sheets of paper 101 by a pressure roller of the fixing unit, the height “h” of a curl at the left and right end portions 101a possibly reaches as much as 30 mm. In addition, the cause contributing to the nonuniformity of moisture brings about complex curls on the sheet of paper. Furthermore, the height of curl tends to depend significantly on the characteristics of the sheet of paper such as the direction of fibers in addition to the amount of moisture contained in the sheet of paper.
It has been likely that local curling of the sheet of paper not only shuts the paper outlet, remarkably damaging the performance in terms of arrangement and stacking of paper discharged onto the discharging tray, but also pushes in the discharging direction of paper out to drop sheets of paper previously stacked off the discharging tray. In this manner, curling of the sheet of paper has more than a few unfavorable effects on the stacking of discharged paper. On the other hand, there is a fact that the occurrence of curling of paper having undergone fixation is unavoidable for an image forming apparatus utilizing electrophotography, which requires a fixing process in which heat and pressure are applied to a sheet of paper during an image forming process. In addition, another image forming apparatus utilizing another type of image forming processes differing from electrophotography possesses a common problem associated with the deterioration in stacking of discharged paper on the discharging tray due to curling of the sheet of paper having traveled through the image forming apparatus, although the amount of curling and the cause for it differ from those of sheets of paper having undergone electrophotography processes.
Various approaches for correcting curling of discharged paper have been proposed. For example, an image forming apparatus employing a sheet member is known, having the following features: The sheet member depresses a stack of paper previously discharged such that the top of the stack of paper is positioned below a front end of a sheet of paper subsequently discharged from a discharge roller. And the sheet member moves with aid of the front end of the sheet of paper, thereby allowing the sheet of paper to be discharged onto a discharging tray. In addition, an image forming apparatus is known, which includes a curl correction member having a shape of lever that corrects curling of a sheet of paper discharged onto a job separator tray.
On the other hand, an image forming apparatus satisfying a recent demand for saving space is known, which is provided with a post-processing unit in an internal paper discharging space that performs stapling for a stack of paper having undergone an image forming process.
In the image forming apparatus provided with the post-processing unit in the internal paper discharging space, a processing tray of the post-processing unit may be adapted to be vertically pivotable to allow recovery from jamming inside the post-processing unit and maintenance of the post-processing unit to be performed. In the above mentioned image forming apparatus, a unit main housing and the processing tray are selectably set in a fixed position or a released position, by allowing a lock member which engages with an engaging hole provided in the unit main housing to project from or retract into an end surface of the processing tray. On the other hand, for a configuration provided with the curl correction member described above, it is necessary to allow the curl correction member to project into the internal paper discharging space such that the curl correction member sufficiently depresses the top surface of sheets of paper discharged into the internal paper discharging space.
For these reasons, it is likely that the lock member projecting from the end surface of the processing tray interferes with the curl correction member while the processing tray is pivoted for recovery from jamming and the like, resulting in damaging of the curl correction member and/or the lock member. In addition, it poses a problem that if a portion of the processing tray is adapted to be pivotal such that the lock member and the curl correction member do not interfere with each other, the workability of performing recovery from jamming deteriorates due to the pivotal portion of the processing tray being apart from where a sheet of paper is introduced. Furthermore, although it may be possible to shift the position of the post-processing unit in a discharging direction of paper, this leads to an increase in the dimension of the image forming apparatus with respect to the discharging direction of paper, thereby resulting in a disadvantage if downsizing and compactness of the image forming apparatus are considered.